Information is stored in disk files in radial tracks defined in the disk surface as is well known. The writing of information into those tracks and the reading of information from those tracks is carried out by a set of read-write heads which are moved to a selected or target track in response to positioning commands from a computer controlling the disk drive. Two types of head positioning systems are used. One uses a dedicated servo surface; the other, an imbedded system as is well understood. The description will be rendered herein in terms of a dedicated servo system.
The set of read-write heads are mounted rigidly on an actuator responsive to computer commands. The actuator responds to position the heads correctly with respect to the target track. The quality of a disk file is determined, to a large extent, by the time it takes, on the average, to reposition the heads at a newly selected position once a command is given.
Typical commercially available disk drives include servo loops operative to control the movement of the actuator. The purpose of the servo loop is to perform a seek function followed by a tracking function in response to a "new track" or target track position command. Once the actuator reaches the target position, the servo searches for the target track center.
Two separate servo loops are involved; the seek servo and the tracking servo. The seek servo operates to move the actuator as fast as possible (maximum velocity) to the target track in a manner so that the actuator moves at zero velocity when it reaches the target track center. The seek servo basically determines the disk drive's most important specification, "ACCESS TIME" and "SINGLE TRACK SEEK TIME". The present invention is directed at achieving relatively short access times especially for the single track seek specification.
Prior art seek servo systems operate as follows: The actuator is moved at maximum velocity until the position error reaches a preselected value. Thereafter, the velocity is controlled, according to a formula, which reduces the velocity from the maximum velocity in a manner proportional to the position error.
An inner velocity loop is provided for controlling the estimated actuator speed with respect to the computed speed as defined by the formula. This inner loop is called "the first order proportional velocity servo loop". The loop operates to amplify the velocity error by the proportional gain to control the estimated feedback to the actuator. Because the frictional term is present and is normally not a constant, and because a finite error gain exists, there will always be a finite velocity error which has to be as large as possible for the system in order to ensure that the velocity approaches zero at the target track. In order for the velocity to approach zero, the feedback also must approach zero so that when the switch from the seek servo to the tracking servo occurs, the actuator will not overshoot the track center causing long settling times or causing the target track to be missed altogether.
In practice, the actuator is positioned by a motor, the current in which is controlled as described above. The amplitude of the motor current (and thus the velocity of the actuator) is proportional to the distance between the start position and the target position for the head (i.e. the position error). In practice, prior art systems generate a "position signal" from the servo head. A derivative of the position signal is calculated and a velocity signal is generated from the derivative. The current which is generated in the actuator motor is proportional to the velocity error signal. The closer the actuator is to the target track, the smaller the velocity signal and the lower the current in the actuator motor.
But the motor current control signal is actually a velocity error times gain signal. Although, the velocity signal amplitude approaches zero as the actuator approaches the target track, the gain increases also approaching an infinite value. The resulting positioning control is thus nonlinear and produces a relatively long access time.
The present invention is directed at providing a disk drive with a relatively short access time and an improved switch from the seek to the tracking servo.